Along with the development of display technologies, flat-panel display device has become a mainstream of the display devices due to such features as being light and thin. Among various flat-panel display devices, OLED display devices and active matrix/organic light-emitting diode (AMOLED) display devices have been widely used nowadays.
For the OLED display device, a light beam from a light-emitting layer is transmitted to the outside through various layers in the OLED display device. The light beam is refracted when it passes from one medium into another medium. FIG. 1 is a partial, schematic view showing the OELD element. The OLED display device includes a substrate 101, and an electrode 102 and a pixel-defined layer 103 arranged on the substrate 101. The pixel-defined layer 103 is provided with a plurality of openings at the substrate 101, so as to expose the electrode 102 and form a predetermined pattern. A light-emitting layer 105 is arranged at each opening. However, when the light beam from the light-emitting layer 105 reaches the pixel-defined layer 103, total reflection will occur at an interface between the pixel-defined layer 103 and air due to a significant difference between refractive indices of the pixel-defined layer 103 and air. As a result, the light beam is constrained within the pixel-defined layer 103 and cannot be used for display.
Hence, there is a remarkable light loss inside the OLED display device, when the light beam passes from one medium to another medium. To be specific, the light loss at an interface between ITO and glass occupies about 50%, and the light loss at an interface between the glass and air occupies about 30%. In this regard, even if the internal quantum efficiency is up to 100%, the light output efficiency is merely 20%. How to increase the light output efficiency has become a critical factor for improving the external quantum efficiency of the OLED.
The light loss also occurs in an AMOLED display device with a top-emission structure when the light beam is transmitted through the interface between different media.